1. Field of the Invention
The present invention relates to a plastic cap of the type of screw engagement. More specifically, the invention relates to a plastic cap of the type of screw engagement in which an inner ring is formed on the inner surface of the top panel of the cap for forming a seal upon coming in intimate contact with the inner surface of the mouth-wall of the container to accomplish a favorable sealing relying upon the inner ring while providing venting in case the pressure in the bottle is elevated.
2. Description of the Related Art
Plastic caps of the type of screw fitted to the mouth-wall of the container relying on the screw engagement have been widely used, for example, for beverage containers. Usually, an inner ring is formed on the inner surface of the top panel of the cap, and an upper end portion of the mouth-wall of the container enters into between a skirt of the cap and the inner ring in a state where the cap is closed, whereby the inner ring comes into intimate contact with the inner surface of the mouth-wall of the container thereby to form a seal.
The cap of the above structure features a high degree of sealing owing to the inner ring leaving, however, a problem to some extent concerning the venting. That is, when the cap is used for the bottles containing fruit juices, it may happen that the cap pops up from the bottle-mouth due to an increase in the pressure in the bottle that results from fruit juices undergoing the secondary fermentation and producing a carbonic acid gas (this often occurs when a bottle containing fruit juices that is left without all consumed is sealed with the cap again and is left to stand for extended periods of time).
A cap which is free from the above problem and offers an improved gas venting has been proposed in, for example, JP-A-2004-224371 having a coupling portion (reinforcing rib) formed from the inner surface of the inner ring through up to the inner surface of the top panel of the cap.